The present invention relates to a composite sorting apparatus that separates a mixture into metal pieces (conductive materials) and plastic pieces (non-conductive materials).
Sorting apparatuses that sort a mixture into metals as conductive materials and plastics as non-conductive materials using electric force include an electrostatic type and a corona discharge type, as well as a composite type using both the electrostatic type and the corona discharge type.
As shown in FIG. 9, this composite sorting apparatus is composed of a specified amount supplying section 53 formed of a hopper 51 and a supply plate 52, a metal drum electrode 54 having a cylindrically formed surface and rotated in a predetermined direction (shown by arrow a) around a horizontal axis, a linear electrode 55 for corona discharge which is provided obliquely above the drum electrode 54 at a downward rotation side thereof and located opposite the drum electrode 54 at a predetermined distance therefrom, a plate-shaped electrode 56 arranged downstream of the linear electrode 55 and opposite the drum electrode 54 at a predetermined distance therefrom to form an electrostatic field, a power supply device 57 that applies high voltage between the drum electrode 54 and both the linear electrode 55 and the plate-shaped electrode 56, and a collecting container 58 arranged below the drum electrode 54 to collect sorted materials therein.
With this construction, the drum electrode 54, rotated in a predetermined direction, is grounded to act as a positive electrode, while the linear electrode 55 is used as a negative electrode, to subject the gas in non-uniform electric fields to corona discharge on the basis of the impact ionization action of electrons, thereby generating negative corona ions. The negative corona ions are applied to the drum electrode 54, and the plate-shaped electrode 56 is used as a negative electrode to form sorting electrostatic fields between the plate-shaped electrode 56 and the drum electrode 54. In this state, a mixture of metal pieces as conductive materials and plastic pieces as non-conductive materials is loaded from the hopper 51 onto the drum electrode 54 via the supply plate 52. Then, the mixture moves toward the downstream side of the drum electrode 54 in a rotating direction thereof as the drum electrode 54 rotates, while negative corona ions from the linear electrode 55 are applied to the metal and plastic pieces. The metal pieces, to which the corona ions have been applied, come into contact with the drum electrode 54, so that negative charges provided by the corona ions are neutralized by positive charges from the drum electrode 54. The drum electrode 54 further provides positive charges to the metal pieces. Thus, the metal pieces repel the drum electrode 54 and fall therefrom. On the other hand, the plastic pieces, to which the corona ions have been applied, are attracted to the drum electrode 54 due to negative charges provided by the corona ions.
Furthermore, in sorting electrostatic fields, the metal pieces, having positive charges, are attracted to the plate-shaped electrode 56 as a negative electrode, whereas the plastic pieces, having negative charges, are attracted to the drum electrode 54 due to electrostatic force acting thereon.
Thus, the mixture is separated into metal pieces and plastic pieces, which are then collected in the collecting container 58 located below the drum electrode 54.
However, with the conventional composite sorting apparatus, the linear electrode 55 that applies corona ions to the mixture of metal pieces and plastic pieces emits only a small amount of corona ions and non-uniformly applies corona ions to the mixture. Accordingly, a sufficient amount of corona ions cannot be applied to the mixture, thereby precluding the mixture from being precisely separated into the metal pieces and the plastic pieces.
Thus, the present invention solves the above problems, and it is an object thereof to provide a composite sorting apparatus that can precisely separate a mixture into metal pieces and plastic pieces.
To solve the above problems, the present invention provides a composite sorting apparatus comprising a rotationally moving electrode disposed so as to rotationally move in a predetermined direction, a discharge electrode for corona discharge which is provided opposite the rotationally moving electrode at a predetermined distance therefrom, a plate-shaped electrostatic electrode provided downstream of the discharge electrode and located opposite the rotationally moving electrode at a predetermined distance therefrom to form an electrostatic field between the electrostatic electrode and the rotationally moving electrode, wherein high voltage of a polarity opposite to that of the rotationally moving electrode is applied between the rotationally moving electrode and both the discharge electrode and electrostatic electrode, a mixture of metal pieces and plastic pieces is loaded onto the rotationally moving electrode, corona ions are applied to the mixture from the discharge electrode, and the mixture is introduced into said electrostatic field so as to be separated into the metal pieces and the plastic pieces, the apparatus being characterized in that the discharge electrode has a plurality of discharge sections each having a sharp tip, the discharge sections are provided in a cross direction of the rotationally moving electrode at predetermined intervals, and arranged so that an interval X cm (in) between adjacent discharge sections with respect to a distance L1 in cm (in) from tips of the discharge sections to the rotationally moving electrode meets the following expression (1), the discharge sections are each formed so that an area formed on the rotationally moving electrode and to which corona ions are applied has a diameter three times as large as the distance L1 in cm (in), and a voltage V1(kV) applied between the discharge electrode and the rotationally moving electrode meets the following expression (2):
0 less than X/L1xe2x89xa63xe2x80x83xe2x80x83(1) 
0.5 kV/cm (1.27 kV/in)xe2x89xa6V1/L1xe2x89xa610 kV/cm (25.4 kV/in)xe2x80x83xe2x80x83(2). 
With respect to the above construction, it has been found that corona ions generated by the discharge electrode having discharge sections each having the sharp tip mostly come from the tip of the discharge section. It has also been found that in a facility environment used for the sorting apparatus, the discharge sections of the discharge electrode each form an ion applied area having a width three times as large as the distance L1 from the tip of the discharge section and the rotationally moving electrode. Accordingly, by setting the interval X between the adjacent discharge sections equal to a value that meets the expression (1), areas to which corona ions from the discharge sections are applied are located at least in contact with each other. Consequently, corona ions are applied to the entire area of the rotationally moving electrode in its cross direction, to ensure a sufficient corona-ion-applied time required for separation, thereby improving separation precision.
Furthermore, the voltage applied per 1 cm (0.3937 in) in the distance between the discharge electrode and the rotationally moving electrode is between 0.5 kV/cm (1.27 kV/in) and 10 kV/cm (25.4 kV/in), thereby ensuring that a sufficient amount of corona ions are generated and preventing the occurrence of a spark (short circuit), which precludes the generation of corona ions. Thus, an amount of corona ions required for separation can be generated.
Accordingly, a mixture of metal pieces and plastic pieces can be precisely separated into the metal pieces and the plastic pieces.
Further, a second aspect of the present invention is the above construction, characterized in that a plurality of rows each composed of a plurality of discharge sections arranged in the cross direction of the rotationally moving electrode are arranged in a rotational-movement direction of the rotationally moving electrode, and a distance D in cm (in) between the rows of discharge sections is set to meet the following expression (3):
D less than 3v+3L1xe2x80x83xe2x80x83(3) 
where v is the circumferential speed in cm/sec in/sec of the rotationally moving electrode.
With the above construction, the distance D between the rows of discharge sections arranged in the rotational-movement direction of the rotationally moving electrode is smaller than the distance that the mixture moves through the corona-ion-applied areas of the discharge section rows during +3 seconds. As a result, the time for which no corona ions are applied to the mixture is limited to shorter than 3 seconds. This prevents charges provided by corona ions from being released, thereby enabling the mixture to be effectively separated into metal pieces and plastic pieces.
Furthermore, a third aspect of the present invention is the above construction, characterized in that the plurality of rows of discharge sections are formed so that the discharge sections of one row are located offset from the corresponding discharge sections of the adjacent row in the cross direction of the rotationally moving electrode.
In the above construction, since the plurality of rows of discharge sections are formed so that the discharge sections of one row are located offset from the corresponding discharge sections of the adjacent row in the cross direction of the rotationally moving electrode, the mixture, moving as the rotationally moving electrode moves rotationally, spends uniform time in passing through the corona-ion-applied areas in the cross direction of the rotationally moving electrode. Consequently, a uniform amount of charges provided by corona ions are applied to the mixture.
Moreover, a fourth aspect of the present invention is the above construction, characterized in that the electrostatic electrode is formed to have, in a direction orthogonal to the rotational-movement direction of the rotationally moving electrode, a length that is substantially the same as the width of the rotationally moving electrode, the electrostatic electrode is formed to have, in the rotational-movement direction of the rotationally moving electrode, a length that is one-tenth or more of a diameter of the rotationally moving electrode, and a voltage V2(kV) applied between the electrostatic electrode and rotationally moving electrode is set to meet the following expression (4):
0.5 kV/cm (1.27 kV/in)xe2x89xa6V2/L2xe2x89xa610 kV/cm (25.4 kV/in)xe2x80x83xe2x80x83(4) 
where L2 is the shortest distance in cm (in) between the electrostatic electrode and the rotationally moving electrode.
In the above construction, since the electrostatic electrode is formed to have, in the direction orthogonal to the rotational-movement direction of the rotationally moving electrode, the length that is substantially the same as the width of the rotationally moving electrode, a uniform electrostatic field is formed over substantially the entire rotationally moving electrode in its cross direction, thereby applying electrostatic force to the mixture, that is, the plastic and metal pieces, depending on the polarity of the charges and the amount of charges. Further, since the electrostatic electrode is formed to have, in the rotational-movement direction of the rotationally moving electrode, the length that is one-tenth or more of the diameter of the rotationally moving electrode, sufficient time can be used to pass the mixture through the electrostatic field, resulting in precise separation. Furthermore, the voltage applied per 1 cm (0.3937 in) in the distance between the electrostatic electrode and the rotationally moving electrode is set between 0.5 kV/cm (1.27 kV/in and 10 kV/cm (25.4 kV/in), thereby preventing the electrostatic field from having an excessively low intensity, which results in weak electrostatic force applied to the mixture. This prevents separation precision from decreasing and also prevents a decrease in separation precision caused by the lack of an electrostatic field resulting from a spark (short circuit) between the electrostatic electrode and the rotationally moving electrode. Therefore, the mixture can be precisely separated into metal pieces and plastic pieces.
In the present invention, the distance L1 in cm (in) from the discharge section of the discharge electrode to the rotationally moving electrode is shown as the distance from the tip of the discharge section as described above, but the distance D between the plurality of discharge section rows arranged in the rotational-movement direction of the rotationally moving electrode is also shown as the distance between the tips of the discharge sections forming the adjacent rows.
Further, when the plurality of rows of discharge sections are formed so that the discharge sections of one row are located offset from the corresponding discharge sections of the adjacent row in the cross direction of the rotationally moving electrode, the discharge sections are arranged so that segments joining the discharge sections forming one row to the corresponding discharge sections forming the adjacent row in the cross direction of the rotationally moving electrode are in what is called a zigzag form.
Further, the length of the electrostatic electrode in the rotational-movement direction of the rotationally moving electrode has no particular upper limit. If this length is too large, the mixture of metal pieces and plastic pieces may bounce of f the electrostatic electrode, so that the sorted plastic pieces may be mixed with metal pieces. This may reduce the purity with which the plastic pieces are sorted out or the recovery of the plastic pieces or the like. Thus, in a practical sense, this length is preferably eight-tenths or less of the diameter of the rotationally moving electrode.